Monolithic semiconductor X-ray detectors comprise, as illustrated in FIG. 1, a planar substrate 1 made of a semiconductor. The detector material may be from the family of detectors that are semiconductors at room temperature. Mention will be made, notably, of CdTe, CdZnTe, GaAs, TlBr, HgI2 or CdMnTe.
This substrate 1 comprises, on one of its sides, a matrix of first, positively biased electrodes 2 and, on the opposite side, a matrix of negatively biased electrodes 3. Each pixel thus consists of an elementary anode connected to its processing electronics. The typical pixel size, defined as the distance over which the electrodes repeat, may range from a few tens of microns to a few hundreds of microns.
When a photon X, the energy of which is located in the energy band from a few keV to a few hundred keV, crosses a semiconductor pixel it creates a number of electron-hole pairs 5 by ionizing atoms in the semiconductor crystal. These charges are captured by the electrodes of the pixel and generate, during their transit, electrical pulses at these electrodes. These pulses are counted by an ASIC 4—“ASIC” standing for application specific integrated circuit. The material 6 used to interconnect the substrate and the ASIC depends on the pixel size. Advantageously, indium or any metal having a low melting point, typically less than 150° C., is used for small sizes or conducting polymer adhesives are used for larger sizes.
For a radiation detection matrix using the counting principle, the pulse emitted by the elementary pixel is amplified, then compared to a threshold in order to decide whether the pulse is counted or not. The amplitude of this threshold defines the pulse amplitude and therefore theoretically the energy delivered to the detector by the incident photon. In the case of a multi-energy counting system, the various thresholds are adjusted so that they each correspond to a precise energy. Thus, energy bands and images corresponding to these bands are defined. These images are notably used in devices for imaging contrast agents or various tissues or for detecting explosives in the case of luggage inspection.
For a matrix of a few thousand pixels, the response sensitivity of the counting system as a function of the position of this threshold may vary quite significantly from one pixel to another, leading to response inhomogeneities. Various methods are commonly used to adjust these thresholds.
For example, to compensate for a temperature drift in the counting electronics, a calibrated charge is injected into all the pixels and the adjustment is carried out by sweeping their thresholds. The thresholds are then adjusted so that finally all the pixels only retain pulses having an amplitude which exceeds that of the injected signal. This injection may be carried out at the pixel level by a dedicated electronic device. It may also be carried out via the capacitor C of the semiconductor detector connected to each pixel. It may, for example, be a CdTe detector connected by an indium bump to a counting ASIC. The semiconductor detector then consists of a bias electrode on the top side and on the bottom side of many electrodes connected to the readout ASIC, pixel by pixel. By applying a rapid voltage variation δV to the common top electrode, an electrical pulse is created, the amplitude of which is C. δV/δt, in each small electrode connected to the input of the amplifier of the counting ASIC. One embodiment is presented in FIG. 2. The series of electronics located at the output of the pixel 10 comprises:                a capacitor 11 enabling an amount of charge to be injected into the amplifier channel so as to calibrate the system;        a first amplification stage 12 that amplifies the charge packet coming from the detector 10 by making, in the case of the figure, a current-voltage conversion;        electronic means 13 enabling the signals to be temporally shaped;        electronic means 14 for defining, with an analog/digital converter, a voltage threshold, used by the comparator;        electronic means 15 for comparing the input voltage with the threshold voltage and delivering a logic signal when this input voltage is greater than the threshold; and        electronic means 16 for counting logic pulses in a given time interval and then transmitting the result to a readout bus.        
The device operates as follows. A voltage of a few volts is applied upstream of the amplification stage 12 located at the output of the pixel 10 for a few nanoseconds to a few tens of nanoseconds, thus simulating the signal produced by an interaction of a photon of a certain energy in the detector. Next, the amplitude of the signal delivered by the various means making up the readout electronics of the pixel is determined, resulting in the amplitude corresponding to the pulse “injected” upstream of the pixel. By carrying out this operation for all the pixels of the detector, it is possible to know, for each pixel, the amplitude corresponding to this given pulse. This amplitude is then considered to be the threshold of each pixel. A detector, in which each pixel has been “thresholded”, is then obtained, each threshold corresponding to the same pulse and therefore to the same energy deposited in the detector.
However, this calibration method has certain drawbacks since the correlation between the energy deposited in the detector and the duration and intensity of the pulse is difficult to establish. In addition, such a method does not take into account differences in charge transport and collection between each pixel. Indeed, two pixels may possess identical electronic capacities but completely different charge transport properties. In addition, the electronic circuits, and in particular the readout circuits, are subject to thermal drift. A threshold corresponding to a given energy may correspond, after a certain time, to a different energy. This relatively tedious operation must therefore be repeated over the course of time.
To carry out a more reliable calibration, it is necessary to use an X-ray or gamma-ray source of known characteristics. In order for the amplitude of the pulses produced by a detector to correspond to the corresponding energy of the incident photons, it is possible to calibrate the system with monoenergetic radiation sources of known energy. These sources are generally not very active and the statistics are insufficient even with long acquisition times. In the case of an X-ray tube, the energies emitted present an energy continuum between a minimum energy defined by the filtration of the generator, about 10 to 20 keV, depending on the application, and a maximum energy defined by the high voltage of the X-ray generator. It is possible to use the maximum energy as reference for the calibration. However, this is not easy when the thresholds must be adjusted to low-energy values, between 10 and 40 keV for example, since the number of photons emitted by the generator in this energy range is then very small. In addition, the generators used are not designed to emit at voltages located in the range from 10 to 40 kV. This is the case for example for X-ray scanners. Another method, described in the U.S. Pat. No. 7,479,639, consists in using radiation sources of well-known energy to calibrate the thresholds. However, this method is long and requires radiation sources to be present in the vicinity of the radiological system. The question of managing these sources in order to meet various safety standards then arises, all the more so in that the most active sources possible must be used if it is desired to achieve a correct calibration in a reasonable period of time.
International patent application WO 2009/122317 provides a calibration device using the radiation source of the detection system. However, this device requires a material of known spectral radiance in order to carry out the calibration. Attention is drawn, in particular, to FIG. 1 in the above application.